The present invention relates to a magnetic recording medium and a process for producing the magnetic recording medium, and more particularly, to a magnetic recording medium having an excellent surface smoothness without deterioration in coercive force thereof and a process for producing the magnetic recording medium.
In recent years, in magnetic recording apparatuses such as a hard disc drive, there has been a remarkable tendency that information devices or systems used therefor are miniaturized and required to have a high reliability. With such a recent tendency, in order to deal with a large capacity data, there is an increasing demand for providing magnetic recording media on which information can be stored with a high density.
For satisfying such requirements, the magnetic recording media have been strongly required to not only have a large coercive force, but also reduce a distance between a magnetic head and a magnetic recording layer (magnetic spacing).
As magnetic recording media having a large coercive force, there is widely known those comprising a substrate and a magnetic thin film formed on the substrate.
The magnetic thin films which have been already put into practice, are generally classified into magnetic iron oxide thin films composed of maghemite, etc. (refer to xe2x80x9cTechnical Report of Electronic Telecommunication Institutexe2x80x9d, published by Electronic Telecommunication Institute, (1981) MR81-20, pp. 5 to 12, xe2x80x9cCeramicsxe2x80x9d, published by Japan Institute of Ceramics, (1986) Vol. 24, No. 1, pp. 21 to 24, and Japanese Patent Publication (KOKOKU) Nos. 51-4086(1976) and 5-63925(1993)); and magnetic alloy thin films composed of Coxe2x80x94Cr alloy; or the like.
The magnetic iron oxide thin films composed of maghemite are excellent in oxidation resistance or corrosion resistance due to inherent properties of the oxides. Therefore, the magnetic iron oxide thin films can show an excellent magnetic stability independent of change in passage of time and less change in magnetic properties with passage of time. Further, since oxides exhibit a higher hardness than that of metals, no protective film is required so that the magnetic spacing of magnetic recording media having such a magnetic iron oxide thin film can be reduced as compared to that of magnetic recording media having the magnetic alloy thin film composed of Coxe2x80x94Cr or the like. Therefore, the magnetic iron oxide thin film is optimum for the production of high-density magnetic recording media.
It has been attempted to enhance a coercive force of maghemite thin films by incorporating cobalt thereinto. However, with the increase in cobalt content, the maghemite thin films tend to be deteriorated in magnetic stability independent of change in passage of time due to adverse influences of heat or the like.
Meanwhile, the present inventors have already proposed a maghemite thin film which can exhibits a high coercive force even with a less cobalt content by controlling the specific plane spacing of maghemite crystal (Japanese Patent Application Laid-Open (KOKAI) Nos. 11-110731(1999) and 11-110732(1999)).
On the other hand, the magnetic alloy thin films composed of Coxe2x80x94Cr or the like, have a coercive force as high as not less than about 2,000 Oe. However, these magnetic alloy thin films are readily oxidized in themselves and, therefore, tend to be deteriorated in stability independent of change in passage of time as well as magnetic properties. In order to prevent the deterioration of magnetic properties due to the oxidation, a protective film of diamond-like carbon, SiO2 or the like having a thickness of usually 100 to 200 xc3x85, is formed on the surface of the magnetic alloy thin film, thereby causing the increase in magnetic spacing corresponding to the thickness of the protective film.
In magnetic recording media, in order to reduce the magnetic spacing, it is necessary to reduce the fly-heights of magnetic head and always cause the magnetic head to be flying stably. In conventional hard disc drive devices, magnetic recording media used therefor have been required to have some surface roughness in order to prevent the magnetic head from being absorbed onto the magnetic recording medium surface due to a meniscus force upon landing on the surface thereof. At the present time, due to the improvement in hard disc systems, magnetic recording media have been no longer required to show such a surface roughness for preventing the magnetic head from being absorbed thereonto. Rather, it has been required that magnetic thin films used in these magnetic recording media have a more excellent surface smoothness.
Also, it is known that the non-smooth surface of magnetic recording media causes media noise. In order to eliminate such a media noise, it is necessary to lessen a surface roughness of the magnetic thin film.
At present, in magnetic recording media having a magnetic oxide thin film, the surface property of the magnetic thin film largely depends on that of an underlying substrate due to its extremely small thickness such as not more than 50 nm. Therefore, it has been required not only to use such a substrate having an excellent surface smoothness, but also to develop techniques for further smoothening the surface of magnetic thin film.
Hitherto, as processes of producing maghemite thin films, there are known (1) a process comprising forming a hematite thin film on a substrate, reducing the hematite thin film at a temperature of 230 to 320xc2x0 C. to transform the hematite thin film into a magnetite thin film, and then oxidizing the magnetite thin film at a temperature of 290 to 330xc2x0 C.; (2) a process comprising forming a magnetite thin film on a substrate and oxidizing the magnetite thin film at a temperature of not less than 320xc2x0 C.; or the like.
Also, as techniques for improving the surface smoothness of magnetic recording media having a magnetic iron oxide thin film, there are known a process of enhancing the surface smoothness under optimized heat- treatment conditions (Japanese Patent No. 2,816,472); a process of sputter-etching the surface of a magnetic recording layer composed of a metal thin film using an oxygen gas (Japanese Patent Application Laid-Open (KOKAI) No. 10-50544(1998)); or the like.
Presently, it has been strongly demanded to provide magnetic recording media having a maghemite thin film capable of showing an excellent surface smoothness while maintaining as much as possible a high coercive force thereof. However, such magnetic recording media satisfying these requirements have not been obtained until now.
Namely, in the process described in Japanese Patent No. 2,816,472, the surface roughness of the glass substrate is reduced by polishing or the like, and the oxidation of the magnetite thin film is conducted in an atmospheric air by appropriately selecting conditions therefor, thereby obtaining magnetic recording media having a low maximum height (Rmax) However, since the thickness of the magnetic thin film is 200 nm in Example of Japanese Patent No. 2,816,472, in the case where the thickness of the magnetic thin film is extremely small, e.g., as small as not more than 50 nm, it is hard to say that the surface roughness of magnetic recording media can be sufficiently improved maintaining as much as possible a high coercive force thereof.
Also, the process described in Japanese Patent Application Laid-Open (KOKAI) No. 10-50544(1998) is directed to the technique for improving the surface smoothness of magnetic recording media having an alloy thin film containing cobalt as a main component. However, in this process, it is required to subject the final product to plasma treatment. Meanwhile, if the maghemite thin film used in the present invention is subjected to the similar plasma treatment, the maghemite thin film suffers from local discoloration, thereby failing to obtain the aimed effect.
As a result of the present inventors"" earnest studies for solving the above problems, it has been found that in the process for producing a magnetic recording medium wherein a magnetite thin film is formed on a substrate and then the magnetite thin film is oxidized to a maghemite thin film, by subjecting the magnetite thin film before oxidized to plasma treatment using an oxygen gas or an oxygen-containing mixed gas, the surface of the obtained maghemite thin film formed on the substrate exhibits a center line average roughness (=an average roughness)(Ra) of 0.1 to 0.7 nm and a maximum height (=a max height) (Rmax) of 1 to 10 nm. The present invention has been attained based on the finding.
It is an object of the present invention to provide a magnetic recording medium which can exhibit an excellent surface smoothness without deterioration in high coercive force thereof.
It is another object of the present invention to provide a economically useful process for producing a maghemite thin film capable of enhancing a surface smoothness of a magnetic layer without deterioration in its high coercive force thereof.
In a first aspect of the present invention, there is provided a magnetic recording medium comprising a substrate and a maghemite thin film formed on the substrate, which maghemite thin film has a thickness of 10 to 50 nm and a surface roughness represented by an average roughness (Ra) of 0.1 to 0.7 nm and a max height (Rmax) of 1 to 10 nm.
In a second aspect of the present invention, there is provided a magnetic recording medium comprising a substrate, a nickel oxide thin film formed on the substrate and a maghemite thin film formed on the nickel oxide thin film, which maghemite thin film has a thickness of 10 to 50 nm and a surface roughness represented by an average roughness (Ra) of 0.1 to 0.7 nm and a max height (Rmax) of 1 to 10 nm.
In a third aspect of the present invention, there is provided a process for producing a magnetic recording medium, comprising the steps of forming a magnetite thin film on a substrate; subjecting the magnetite thin film to plasma treatment using an oxygen gas or an oxygen-containing mixed gas; and oxidizing the thus treated magnetite thin film into a maghemite thin film.